November 10.
I'm using the thick clouds and heavy rain sent from many states away by tropical
storm Ida to prepare a vintage, manual-focus 200mm F2.0 EDIF Nikkor for
action under the stars during the next
clear spell. (Almost exactly one year ago — 11/18/2008 — I was messing
with this glass for daylight use, but now I remember why I bought it.) I
have no way to use H-a filters with this lens on the Canon, though I'm working on that, so
I won't be bucking moonlight with it very soon. Compared to the 70-200mm F4 Canon (stopped to 45mm aperture), this big Nikon lens should collect
over ten times as many photons per second (or, since I won't be selecting only
those that pass through the eye of a spectral needle, about 500 times as many).
This lens is what I had in mind when working up the heavy-duty camera
mount. It's not needed for a lightweight combination like a DSLR with the Canon
70-200mm F4, but the 200mm F2.0 Nikkor is massive. When not used as an astrograph,
you could moor your houseboat to it. Seriously.
This afternoon, I downloaded, tried, and immediately
registered a copy of Neil
Carboni's "Starspikes Pro" plug-in
for Photoshop. This will take some 'splainin', so bear with me.
Whenever you make astrophotos with telescopes having
a secondary mirror suspended in the optical path, bright stars are graced (or
plagued) by "diffraction spikes" because of the way starlight behaves
when it passes the vanes holding the secondary mirror. Really
elaborate crosses and radial patterns can be generated. The same thing happens
when you use a telephoto lens with the diaphragm closed down even a stop or
two: the sharp corners formed by the intersection of the iris blades cause
multi-point "stars" to
form around every point source, whether sun-sparkles, real stars, or catch
lights in your subject's eyes.
These effects are present in a lot of classic astrophotographs
by virtue of the equipment used to make them. The Palomar 200-inch, the Hubble
Space Telescope -- most observatory instruments -- as well as virtually all Newtonian reflectors,
Ritchey-Chretiens, some Schmidt cameras, etc., produce them. Refractors, SCTs, Maks, and wide-open
camera lenses do not. They're part of
a received aesthetic in astrophotography, and they are sometimes beautiful.
But they always
indicate some degree of scattered light and signify compromised image contrast.
Sometimes people stretch wire, string, or tape across the objectives of otherwise
unobstructed telescopes just to produce these effects. This strategem elicits
reactions among cognoscenti ranging from admiration to grudging acceptance
through bemusement and on toward principled outrage. If, as do I, you own and
use a high-end refractor whose maker has gone to great trouble to keep his
handiwork from scattering any light (thank you, Roland)
intentionally scattering some seems anethema.
I should confess: I've done this exactly once. When
I needed a photo of NGC 404 in which the small "Not Found"
galaxy was immediately distinguishable from the nearby stars Beta Andromedae
and HD 6892, I stretched two strips of tape across the dew shield of a 5-inch
Astro-Physics refractor so that the stars, but
not the galaxy, would be marked by diffraction spikes. Worked well, looked
great.
Neil Carboni, who develops and distributes a
set of Photoshop actions specifically for astrophotographers, has written
a sophisitcated and specialized piece of software to mimic these diffraction spikes. The digital analogs (!) are convincing
and they can be tailored in any of several ways to produce exactly the appearance
you want. And, since it's done with post-processing, you don't have to corrupt any
data to get 'em. Take the color shot of the Cygnus starclouds,
for instance. See the previous slowblog entry for the original version, or look here
for the version with starrier stars:
Cygnus in Hydrogen-alpha and
white light.
For all the sordid details, see previous Slowblog entry.
Since I've learned some tricks about
combining data in the course of the previous entry, I've been revisiting some
earlier data collections. Last August, I made a series of 75-second
exposures of Messier
13, the great globular cluster in Hercules. Mostly they were meant to be trials
of focus repeatability and unassisted tracking, but I've combined into
a 20-minute exposure and dressed it up using Neil's latest tool:
Messier 13. 16x75s, ISO 800.
Hap-Griffin modified Canon 20D.
IDAS LPR filter. Astro-Physics 5-inch F6 on Losmandy G11, no guiding.
Raw frames aligned and sigma-combined in Maxim
DL v5,
color data overlayed from a single frame in Photoshop,
diffraction spikes added by Starspikes Pro.
November 11. While
casting about for some new targets, I looked up others' efforts to image
Simeis 147, a huge supenova remnant in Taurus. That brought me to J-P
Metsavainio's
website, AstroAnarchy,
where his spectacular photos make a good case for removing stars from
images to permit aggressive processing of DSO's (Deep Sky Objects). And he
makes available a set of Photoshop actions to accomplish that removal. I'm
looking forward to taking these actions and this technique for a walk.
Simeis 147 has been very well recorded using dedicated CCDs (including by
J-P Metsavainio). Google it. I'm thinking this object is within reach of (mere)
DSLRs under my less than pristine skies. What a world.
November 12. So,
how effective is this remove-the-stars, process-like-there-is-no-tomorrow,
then-put-the-stars-back (RTSPLTINTTPTSB) technique? Very!
This is 100 seconds at an effective F5.6 with the 70-200mm.
70mm, no filters, no stacking, Hap Griffin modified Canon 20D:
This is the same photo after applying the RTSPLTINTTPTSB
strategy (I used the AstroAnarchy action to accomplish the star removal, and after putting the stars back in the firmament, I used Neil Carboni's StarSpike Pro to dress them up for dinner):
This result is a mite heavy-handed — I
just wanted some idea what can be done with this. The word for the week is
"promising." What
happens if you start with good data?
I'm messing with the 200mm F2.0, finding
out how deeply I can bury the objective in a dew shield made from black
Kydex and flocking paper to prevent dew and control scattered light (the
acre of exposed glass calls dew from the driest skies). I'm testing by wrapping
tubes made from card stock around the objective and looking for any signs
of intrusion in the corners. It's beginning to look like the APS-sized
sensors in my Canons (and the even smaller chip in the SBIG ST2000XM) will
make complete control of those two issues easy. Take note of that remark
and prepare to ridicule me mercilessly.
